1. Field of the Invention
This invention relates generally to a very high volt electrolyte for use in electrolytic capacitors and to an electrolytic capacitor impregnated with the electrolyte of the present invention for use in implantable cardioverter defibrillators (ICD). More specifically, the invention relates to the incorporation of a polymer matrix into a standard solvent-based fill electrolyte to raise the breakdown voltage (limit) of the electrolyte to as much as 800 V and, in turn, to a very high volt aluminum electrolytic capacitor impregnated with the electrolyte of the present invention, operating at a voltage of 700 to 800 volts.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
To further reduce the size of the implanted device, there is a need for a single capacitor arrangement for an ICD, capable of operating at a voltage of 700 to 800 volts, which can replace the current two capacitors in series arrangement. However, this has not been possible since available electrolytic capacitor technology has limited photo flash electrolytic capacitor voltages to 600 V and below.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
Conventional electrolytic capacitors that employ a standard solvent-based liquid electrolyte utilize a thick mechanical separator, typically made of kraft paper, that is impregnated with and acts as a reservoir for the electrolyte. However, it has been suggested that by using a polymer based electrolyte, the thickness of the separator material can be greatly reduced and, in some cases, the separator material can be eliminated entirely.
U.S. Pat. No. 4,942,501 and its continuations, U.S. Pat. Nos. 5,146,391 and 5,153,820, each of which is incorporated herein by reference, suggest reducing the volume of electrolytic capacitors by completely eliminating the need for a mechanical separator. They provide an electrolytic capacitor that instead employs, between its anode and cathode, a layer of solid electrolyte comprising a solid solution of a metal salt in a polymer matrix. The preferred method of constructing these capacitors is to deposit a liquid prepolymer electrolyte mixture onto the surface of the anode, and then to cause polymerization to take place to cure the electrolyte. The cathode is thereafter formed by deposition upon the surface of the cured electrolyte layer.
U.S. Pat. No. 5,585,039, incorporated herein by reference, suggests producing a solid polymer electrolyte consisting of the containment of an electrolyte solution within a polymer matrix having a multiphase structure, suitable for use in high energy density batteries, such as lithium batteries. Also disclosed is a method of manufacturing the solid polymer electrolyte comprising the steps of preparing a polymer matrix having a multiphase structure first, followed by impregnating the electrolyte solution into the polymer matrix. Alternatively, the method may comprise the steps of preparing a polymer matrix having a multiphase structure and containing an electrolyte first, then impregnating a solvent into the polymer matrix having the multiphase structure containing the electrolyte.
In known processes for impregnating electrolytic capacitor stacks or wound rolls with solid polymer electrolytes, a polymerization initiator is typically mixed with the electrolyte prior to impregnation. For example, U.S. Pat. No. 5,628,801 discloses an electrolytic capacitor where a solid electrolyte alone or a separator impregnated with an elastomeric solid electrolyte is utilized in the dual capacity of electrolyte and adhesive material to hold together the anode and cathode plates of the capacitor. The preferred electrolyte consists of: 17.5 parts of hydroxyethylmethacrylate, 32.5 parts ethylene glycol, 7.0 parts ammonium adipate, 6.7 parts ammonium glutarate, 0.45 parts tetraethyleneglycoldiacrylate, and 2.2 parts of initiator solution. The preferred initiator solution consists of a solution of 3.6 g of Cu(No.sub.3).sub.2.3H.sub.2 O and 42.4 g of K.sub.2 S.sub.2 O.sub.8 per liter of pure water. The capacitor assembly is impregnated with this polymerizable liquid electrolyte/adhesive and then heated to approximately 55.degree. C. for at least 2 hours, but preferably 24 hours to cure the electrolyte/adhesive.
Similarly, U.S. Pat. No. 5,748,439 discloses an electrolytic capacitor having interposed between the electrically conductive anode and cathode layers thereof a reduced thickness spacer comprised of a mechanical separator means such as kraft paper impregnated with a crosslinked elastomeric electrolyte. The electrolyte is preferably made up as a liquid prepolymer electrolyte mixture prior to impregnation into the capacitor element and the polymer is preferably formed in situ thereafter from the prepolymer mixture. The prepolymer electrolyte mixture is preferably made up by first dissolving a salt into a liquid plasticizer component by stirring at elevated temperatures, cooling the mixture to room temperature, and then adding to the mixture a monomer corresponding to the desired polymer and a crosslinking agent, as well as a polymerization initiator. As a result, the electrolyte acts to strengthen the separator material, allowing a storage device to be constructed with separator materials of reduced thickness.
The problem with the above polymer electrolytes and processes for impregnating electrolytic capacitors with such polymer electrolytes is incomplete filling of the macroscopic tunnels in the etched aluminum anodes. The processes described above suggest combining a polymerization initiator compound with the polymer electrolyte mixture, prior to impregnation, to promote the break down of the ionic salt of the electrolyte mixture. However, when the polymerization initiator is mixed with the polymer electrolyte, polymerization begins, increasing the viscosity of the solution, which reduces the working pot life. Heating the electrolyte mixture to reduce viscosity, a common practice in the industry, only serves to hasten the curing of the polymer and thus defeats the intended purposes. Because of the increased viscosity and the reduced working time, the polymer mixture has insufficient time to fully incorporate itself into the microscopic features of the anode foil. Capacitance is lost due to the incomplete use of the etched foil. Consequently, such capacitors have a breakdown voltage of less than 700 volts. Thus, there is a need for an improved electrolyte and impregnation process which solves these problems.